Modern electronic circuits often have analog and digital portions integrated on a single chip. Such circuits ("mixed-signal chips") are, for example and not intended to be limiting, analog-to-digital converter (ADCs) or digital-to-analog converters (DACs). Unwanted noise signals, such as common mode spikes can parasitically leak from digital portions to analog portions. It is well known in the art to lower the spike sensitivity of the analog portions by a differential design. This approach includes differential current sources supplying currents, for example, I.sub.out1 and I.sub.out2 to the electronic circuits. The magnitude of currents I.sub.out1 and I.sub.out2 and their ratio I.sub.out1 /I.sub.out2 should be substantially temperature invariant.
The differential current sources are, preferably, implemented by transistor pairs. For example, a first transistor provides I.sub.out1 and a second transistor provides I.sub.out2. Field effect transistors (FETs) are often used. The transconductances g.sub.m1 of the first transistor and g.sub.m2 of the second transistor depend among others on the physical dimension of the transistors (e.g., channel width W and channel length L) and on the availability and mobility of charge carriers (e.g., electrons and holes). Changes of the environment temperature influence the transconductances g.sub.m1 and g.sub.m2 and therefore the magnitude of currents I.sub.out1 and I.sub.out2 as well as their relation. Such temperature influences are not wanted and should be compensated. Relations between g.sub.m, W, L, the temperature and other parameters are analytically expressed by well known equations. For example, equations and more details for FETs are given in: Horowitz, P., Hill, W.: "The Art of Electronics", Second Edition, Cambridge University Press, 1990, ISBN 0-521-37095-7, chapter 3.07 "FET amplifiers" on pages 129-133 [1]. One can expect that the transconductances of two non-ideal transistors differ even if the applied currents or voltages are equal.
Spikes going from the digital portion to the transistor pair should affect the differential currents in a common mode rather than in a differential mode. Temperature dependent changes of the transconductance ratio g.sub.m1 /g.sub.m2 would have an unwanted effect on the response of the transistor pair to a spike at different temperatures. For example, at some temperatures, differential mode responses could prevail over common mode responses, or at different temperatures, the common mode responses prevail.
Modem manufacturing processes for electronic circuits allow one to decrease operating voltages so as to become substantially as low as transistor threshold voltages ("low voltage"). Also, the dimensions are shrunk ("deep sub-micron technology"). This lead among other things to a higher complexity of equations involving transconductances and bias voltages. Under such conditions it is difficult to predetermine the transconductances in the design process and to reproduce transconductances during manufacturing.
Prior art current sources are described in U.S. Pat. Nos. 5,408,235 to Doyle et al. (especially in FIG. 13) [2] and 5,223,743 to Nakagawara [3]. The present invention seeks to provide current sources which mitigate or avoid these and other disadvantages and limitations of the prior art.